1. Field of the Invention
The present invention is related to a wireless communication system, more particularly to a method of handling a radio link failure in a multiple-carrier system.
2. Related Art
Wireless communication systems are widely spread all over the world to provide various types of communication services such as voice or data. In general, the wireless communication system is a multiple access system capable of supporting communication with multiple users by sharing available radio resources. Examples of the multiple access system include a time division multiple access (TDMA) system, a code division multiple access (CDMA) system, a frequency division multiple access (FDMA) system, an orthogonal frequency division multiple access (OFDMA) system, a single carrier frequency division multiple access (SC-FDMA) system, etc.
In the wireless communication system, one carrier is considered in general even if a bandwidth is differently set between an uplink and a downlink. In 3rd generation partnership project (3GPP) long term evolution (LTE), one carrier constitutes each of the uplink and the downlink on the basis of a single carrier, and the bandwidth of the uplink is symmetrical to the bandwidth of the downlink. However, except for some areas of the world, it is not easy to allocate frequencies of wide bandwidths. Therefore, as a technique for effectively using fragmented small bands, a carrier aggregation technique is being developed to obtain the same effect as when a band of a logically wide bandwidth is used by physically aggregating a plurality of bands in a frequency domain. The carrier aggregation includes a technique for supporting a system bandwidth of 100 mega Hertz (MHz) by using multiple carriers even if, for example, the 3GPP LTE supports a bandwidth of up to 20 MHz, and a technique for allocating an asymmetric bandwidth between the uplink and the downlink.
Meanwhile, the user equipment (UE) may be in a radio resource control (RRC) connected mode or in an idle mode. When the UE is in the RRC connected mode, a radio link between the UE and the BS is connected. However, when a channel state deteriorates, out-of-synchronization of a radio link may be generated from a physical layer of the UE. When the out-of-synchronization takes place consecutively more than a certain number of times, the UE declares a radio link failure (RLF). If no carrier aggregation is assumed, the UE detects out-of-synchronization indications only for one carrier. However, when carrier aggregation is used, multiple aggregated carriers are configured for one UE. Thus, when out-of-synch indications occur from multiple carriers, UE cannot properly declare radio link failure.